1. Field of the Invention
The present invention relates to a novel method of fabricating an organic electroluminescent (hereinafter referred to as "EL") display panel comprising organic EL elements which emit light when electric charges are injected into them.
2. Background of the Related Art
The technology of organic EL devices, also called organic light emitting diodes (LEDs), has been rapidly advancing, and several prototype modules have been successfully demonstrated. Organic EL devices are extremely thin, matrix-addressable and are operable at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional features suitable for next generation flat panel displays (FPDs). Some of these features include a low dependence on viewing angle and good device-formability on flexible substrates. A major drawback of liquid crystal displays, which are the most common display of choice, is that most of them require bright backlighting. The backlighting requirement can be easily eliminated by the use of an organic EL display.
Organic LEDs differ fundamentally from conventional inorganic LEDs. While the charge transfer in inorganics is band-like in nature and the electron-hole recombination results in the interband emission of light, organic films are generally characterized by low-mobility activated hopping transport, and excitonic emission. Organic EL devices are also substantially different from conventional inorganic EL devices, especially in that organic EL devices are operable at low DC voltages.
A substantial amount of research has been directed towards improving the efficiency and color control of organic LEDs. The efficiency of organic EL devices has now been demonstrated to be adequate for many commercial applications. Moreover, color control is probably not limiting for most potential applications. Accordingly, the outlook for organic EL devices in commercial applications is excellent. The performance of the organic EL devices is satisfactory for many applications. It is important to consider specific products and manufacturing techniques for the commercialization of organic EL devices. Consideration of specific applications leads us to believe that more work on manufacturability, uniformity, reliability, and systems issues is required to commercialize organic EL devices.
As shown in FIG. 1, the simplest way to drive an organic EL panel is to have organic function layers sandwiched between two orthogonal sets of electrodes, i.e., rows and columns. In this passive addressing scheme, the EL element serves both the display and switching functions. The diode-like nonlinear current-voltage characteristic of the organic EL element should, in principle, permit a high degree of multiplexing in this mode of addressing.
Pixelation or patterning, especially of electroluminescent and second electrode materials, is one of the key issues to be resolved before organic EL devices can be commercialized. The use of many conventional pixelation techniques is precluded due to the nature of organic materials, which are extremely vulnerable to most solvents.
The simplest patterning method is to use a shadow mask. As shown in FIGS. 1 and 2, the pixelation of an organic EL display panel can be accomplished by depositing second electrode material(s) 4 through the openings of a shadow mask 5 onto organic function layers 3 which are, in turn, laminated on a plurality of first electrode stripes 2. The first electrode stripes 2 are generally formed by patterning a layer of indium tin oxide (ITO) deposited on a transparent, insulating substrate 1.
Pixelation using a shadow mask becomes less efficient as the display resolution becomes finer. One possible solution, for a monochrome display, is to separate adjacent pixels using electrically insulating ramparts 6, as suggested in U.S. Pat. No. 5,701,055 and as shown in FIG. 3.
As disclosed in JP Laid Open Patent No. H8-315981, and as shown in FIGS. 4a-4d, the use of additional shadow masks may be required for the construction of a multi or full color display. The method disclosed in JP Laid Open Patent No. H8-315981 comprises: (1) putting a shadow mask with a plurality of openings (5-1, 5-2 or 5-3) onto top surfaces of the ramparts 6, and aligning each of said openings so they are over the gap between corresponding ramparts; (2) depositing organic EL medium layers of red (R) 4-1, green (G) 4-2 and blue (B) 4-3, one by one, through the openings 5-1, 5-2 and 5-3 respectively; and (3) forming at least one additional electrode layer 3 on said ramparts and said organic function layers.
The above method may work fine for a display of moderate resolution and size. However, as the display size increases and the pitch decreases, the above-described method reveals some limitations, i.e., difficulties in making the shadow mask itself, and difficulties in aligning the shadow mask with respect to the substrate. The need to accomplish pixelation without resorting to one or more shadow masks has been, in part, addressed in U.S. Pat. No. 5,693,962 and JP Laid Open Patent No. H9-293589.
FIGS. 5a-5c illustrate the fabrication steps of a full color panel, as disclosed in U.S. Pat. No. 5,693,962. FIG. 5a describes the formation process of a first sub-pixel 15-1, which comprises the steps of: (1) patterning a layer of organic or inorganic conductor deposited on a transparent substrate 11, by conventional lithographic techniques, to form a plurality of parallel conductive stripes for the first electrode 12; (2) depositing a layer of dielectric medium 13 on top of the conductive stripes 12 and on top of the exposed portions of the substrate 11; (3) spin-coating a photoresist (PR) layer 14-1, and patterning the dielectric medium 13 by a dry or wet etching technique; (4) laminating an electroluminescent medium 16-1; (5) forming a capping layer 17-1 of the sub-pixel by depositing an air-stable metal on top of the electroluminescent medium 16-1; and (6) removing the PR layer 14-1 and those on the PR layer by lift-off.
Acetone or a stripping solution is often used for lift-off. It is important to note that the electroluminescent medium 16-1 can be exposed, through the capping layer 17-1, to the solvents during the lift-off process. The device degradation due to this exposure is not surprising, given the relatively poor adhesion between an organic material and a metal used as a capping layer. As shown in FIGS. 5b and 5c, a second sub-pixel 15-2 and a third sub-pixel 15-3 can be fabricated by the same process.
FIGS. 6a-6j describe a processes of forming R, G, B sub-pixels, as disclosed in Japanese Laid Open Patent No. H9-293589. The process comprises the steps of: (1) forming a first electrode layer 22 on a transparent substrate 21, followed by the lamination of a red (R) electroluminescent medium 23, a second electrode 24, and a protection layer 25; (2) spin-casting a photoresist (PR) 26 and subsequently patterning red sub-pixels; and (3) repeating the above steps to form green and blue sub-pixels. During the above-described processing steps, active display elements are substantially exposed to PR 26, which is detrimental to device performance because of solvents remaining in the PR 26. In order to appreciated the adverse effects of photoresist, it is necessary to understand a typical photolithographic process. A typical photolithographic process involves spin-casting of a photoresist solution on the substrate, soft-baking and exposure to an ultraviolet ray, developing, and hard-baking. In the background art discussed herein, the panel is thereafter supposed to be subjected to etching and then PR stripping, which again requires stripping solution(s). During spin-casting, active EL elements are already exposed to solvents from the photoresist in a liquid phase.
In short, the background methods described above have a serious drawback. Active EL elements are inevitably exposed to various solvents from photoresist or developing and stripping solutions These solvents are extremely harmful to the electroluminescent medium, and adversely affects the device performance.